title: BisGMA/TEGDMA ratio and filler content effects on shrinkage stress.
abstract: OBJECTIVE
  To investigate the contributions of BisGMA:TEGDMA and filler content on polymerization stress, along with the influence of variables associated with stress development, namely, degree of conversion, reaction rate, shrinkage, elastic modulus and loss tangent for a series of experimental dental composites.
  
  
  METHODS
  Twenty formulations with BisGMA:TEGDMA ratios of 3:7, 4:6, 5:5, 6:4 and 7:3 and barium glass filler levels of 40, 50, 60 or 70wt% were studied. Polymerization stress was determined in a tensilometer, inserting the composite between acrylic rods fixed to clamps of a universal test machine and dividing the maximum load recorded by the rods cross-sectional area. Conversion and reaction rate were determined by infra-red spectroscopy. Shrinkage was measured by mercury dilatometer. Modulus was obtained by three-point bending. Loss tangent was determined by dynamic nanoindentation. Regression analyses were performed to estimate the effect of organic and inorganic contents on each studied variable, while a stepwise forward regression identified significant variables for polymerization stress.
  
  
  RESULTS
  All variables showed dependence on inorganic concentration and monomeric content. The resin matrix showed a stronger influence on polymerization stress, conversion and reaction rate, whereas filler fraction showed a stronger influence on shrinkage, modulus and loss tangent. Shrinkage and conversion were significantly related to polymerization stress.
  
  
  CONCLUSIONS
  Both the inorganic filler concentration and monomeric content affect polymerization stress, but the stronger influence of the resin matrix suggests that it may be possible to reduce stress by modifying resin composition without sacrificing filler content. The main challenge is to develop formulations with low shrinkage without sacrificing degree of conversion.
authors.affiliation: Department of Dental Materials, School of Dentistry, University of São Paulo, São Paulo, Brazil.
authors.forename: Flávia
authors.initials: F
authors.lastname: Gonçalves
authors.forename: Caio L N
authors.initials: CL
authors.lastname: Azevedo
authors.forename: Jack L
authors.initials: JL
authors.lastname: Ferracane
authors.forename: Roberto R
authors.initials: RR
authors.lastname: Braga
mesh_terms: D000465:Algorithms; D017609:Barium Compounds / Q000737:chemistry*; D017438:Bisphenol A-Glycidyl Methacrylate / Q000737:chemistry*; D002626:Chemistry, Pharmaceutical; D003764:Dental Materials / Q000737:chemistry*; D003799:Dental Stress Analysis / Q000295:instrumentation; D055119:Elastic Modulus; D006244:Hardness; D006801:Humans; D008422:Materials Testing; D018583:Pliability; D011092:Polyethylene Glycols / Q000737:chemistry*; D058105:Polymerization; D011109:Polymethacrylic Acids / Q000737:chemistry*; D012822:Silicon Dioxide / Q000737:chemistry*; D019265:Spectroscopy, Near-Infrared; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors
chemical_list: D017609:Barium Compounds; D003764:Dental Materials; D011109:Polymethacrylic Acids; C102738:barium glass filler; C020946:triethylene glycol dimethacrylate; D011092:Polyethylene Glycols; D017438:Bisphenol A-Glycidyl Methacrylate; D012822:Silicon Dioxide